A method of manufacturing a cam shaft assembly according to the prior art is as follows.
In Japanese Laid-Open Patent No. 1987-134160, an example in which a cam shaft assembly having a solid shaft is manufactured by casting is disclosed.
In Japanese Unexamined Patent Publication No. 1991-56606, an example in which a cam piece is assembled in a hollow shaft by sintering is disclosed.
In US Patent Publication Nos. 2013-0047944 and 2010-288217, a cam shaft assembly having a sliding cam is disclosed.
In the cam shaft assembly having such a sliding cam, a sliding cam having a plurality of cam tracks cannot be rotated on the cam shaft but can be axially slidably mounted, and the sliding cam has a lifting shape engaged with an operation unit (implemented as a pin) of an actuator so as to achieve axial sliding of the cam. Each valve stroke is set by the axial sliding of the cam.
In such a conventional sliding cam, a hollow tube slidably provided with respect to a shaft, a lift guide disposed in both sides, and a high cam and a low cam which are disposed in close proximity to the lift guides of both sides are integrally formed. Such a sliding cam and a fixed cam are manufactured by machining a raw material in the shape of a round rod to conform to the shape of the cam. Thus, the machining quantities are excessively increased, which causes an increase in material costs, tooling costs, and the like.
In order to solve such a problem, as disclosed in German Unexamined Patent Publication No. 102011001711, an example in which a cam piece and a guide piece are integrally formed, a hollow tubular portion is separately formed, and the integrally formed cam piece and guide piece and the separately formed hollow tubular portion are coupled with each other through forced coupling and shape coupling is shown.
Meanwhile, in a case of the sliding cam, tolerance setting between a cam shaft and the sliding cam is a significantly important factor. Thus, in European Patent Publication No. 60257, an example in which electron beam hardening (EBH) is performed on a material having been subjected to machining to improve the hardness of the surface of the cam so that the deformation is minimized is shown. However, there is a problem that the hardness of a portion of the low cam close to the high cam is not uniform with other portions due to a shadow effect of the high cam portion at the time of EBH heat treatment.